The invention is directed to an integrated circuit package which provides increased strength to surface mount integrated circuit package leads for increased lead dimensional stability as compared to prior art integrated circuit packages. The invention is also directed to methods for making the improved packaging. The invention is also directed to a method for producing a plastic integrated circuit package that allows for transfer molding using a non-conductive, permanent dambar.
In the prior art, the following integrated circuit packages are known:
a. Bare, formed leads with no additional support added.
b. Leadframes that use conductive metal dambars which must be removed before testing and/or mounting.
c. Molded test rings and pre-molded test rings.
When using prior art unsupported formed leads, the leadframe material is necessarily thinner as leads get narrower to accommodate the finer pitches needed for high density integrated circuits, i.e., with leadcounts of 200 or more leads. These higher leadcounts generally require, centerline to centerline pitch of less than 0.025 inch. The reduced cross-section of the leads reduces their strength and increases the likelihood of losing their dimensional integrity.
When dambars are used to provide additional strength when finer pitches are used, removal of the dambars requires mechanical punches to pierce the portions of the dambar in between the leads without damaging the leads. As the pitch between leads decreases, so must the width of the punch. As the punch decreases in width, its strength is reduced and the leadframe material must be reduced to compensate for the reduced strength. The reduction in material thickness reduces the transverse and lateral strength of the leads, making them more prone to damage.
Although lasers are used as another method for removing dambars, this method also requires the leadframe material thickness to be reduced, thereby causing the same lead damage problem as exists when using mechanical punches.
In addition, proper alignment of the leads and dambar punches is critical, especially as the pitch decreases, to insure correct lead widths. Due to tolerance build-up between the leadframe guide holes, alignment pins, and punches, a consistent cross section of the leads is very difficult. This lack of consistency is not only a cosmetic concern, but can also affect the uniform formability of the leads.
Additionally, because of the clearances required between the punches and the leads, a small amount of extraneous material (epoxy material that flows from the body to the dambar in between the leads) will remain after a mechanical deflashing process which is a process to remove the very thin coat of resin that bleeds out onto the surfaces of the leads. Normally, it is removed by chemically dissolving the flash and/or by bombarding the affected surfaces with small plastic beads (called media deflash process). Unless the flash is removed, it is difficult to coat the leads with tin/lead solder which is required for mounting onto printed circuit boards. Not only is this extraneous material a cosmetic concern, it is also a potential source of contamination on the leads or damage to the leads if the extraneous material were to break away during subsequent assembly and test steps.
When a molded test ring is used to provide additional strength when finer pitches are used, multiple metal dambars are also needed, one for the package and two for the ring. These dambars have the same disadvantages as described above.
Additionally, a molded test ring has a different thermal coefficient of expansion than that of the leadframe which is ordinarily made of copper. The difference in the thermal coefficient of expansion leads to flatness issues wherein for electrical testing, test probes (which lie on a plane) make contact with corresponding leads of the package prior to forming for lead pitches less than 0.025 inch center line to center line. For a molded carrier ring, the leads lie on the surface of the carrier ring which generally warps. Consequently, not all test probes are able to make contact with corresponding leads with a reasonable amount of pressure. A large pressure may damage or lower the life of the probes which makes testing at very fine pitches (e.g., less than 0.4 mm) very difficult.
Furthermore, a molded test ring or a pre-molded test ring require the leads to be cut free from the ring before forming free leads. Forming free leads, as opposed to leads still tied together, gives less control over formability and positional integrity.
Another disadvantage to techniques using a molded test ring or pre-molded test ring is that both molded test rings and premolded test rings are added during assembly, after a die has been dedicated to the leadframe. Any yield loss associated with the molding of the test ring or attachment of the pre-molded test ring results in a lost die.